Heavy Mass Loss Research Articles

RAVE spectroscopy of luminous blue variables in the Large Magellanic Cloud

<i>Context. <i/>The RAVE spectroscopic survey for galactic structure and evolution obtains 8400–8800 Å spectra at 7500 resolving power at the UK Schmidt Telescope using the 6dF multi-fiber positioner. More than 300 000 9 <i>≤<i/> <i>I<i/><sub>C<sub/> <i>≤<i/> 12 and |<i>b<i/>| <i>≥<i/> 25° southern stars have been observed to date.<i>Aims. <i/>This paper presents the first intrinsic examination of stellar spectra from the RAVE survey, aimed at evaluating their diagnostic potential for peculiar stars and at contributing to the general understanding of luminous blue variables (LBVs).<i>Methods. <i/>We used the multi-epoch spectra for all seven LBVs observed, between 2005 and 2008, in the Large Magellanic Cloud (LMC) by the RAVE survey.<i>Results. <i/>We demonstrate that RAVE spectra possess significant diagnostic potential when applied to peculiar stars and, in particular, LBVs. The behaviour of the radial velocities for both emission and absorption lines, and the spectral changes between outburst and quiescence states are described and found to agree with evidence gathered at more conventional wavelengths. The wind outflow signatures and their variability are investigated, with multi-components detected in S Doradus. Photoionisation modelling of the rich emission line spectrum of R 127 shows evidence of a massive detached ionised shell that was ejected during the 1982–2000 outburst. Surface inhomogeneities in the nuclear-processed material, brought to the surface by heavy mass loss, could have been observed in S Doradus, even if alternative explanations are possible. We also detect the transition from quiescence to outburst state in R 71. Finally, our spectrum of R 84 offers one of the clearest views of its cool companion.

A top-heavy stellar initial mass function in starbursts as an explanation for the high mass-to-light ratios of ultra-compact dwarf galaxies

It has been shown recently that the dynamical V-band mass-to-light ratios of compact stellar systems with masses from 10^6 to 10^8 Solar masses are not consistent with the predictions from simple stellar population (SSP) models. Top-heavy stellar initial mass functions (IMFs) in these so-called ultra compact dwarf galaxies (UCDs) offer an attractive explanation for this finding, the stellar remnants and retained stellar envelopes providing the unseen mass. We therefore construct a model which quantifies by how much the IMFs of UCDs would have to deviate in the intermediate-mass and high-mass range from the canonical IMF in order to account for the enhanced M/L_V ratio of the UCDs. The deduced high-mass IMF in the UCDs depends on the age of the UCDs and the number of faint products of stellar evolution retained by them. Assuming that the IMF in the UCDs is a three-part power-law equal to the canonical IMF in the low-mass range and taking 20% as a plausible choice for the fraction of the remnants of high-mass stars retained by UCDs, the model suggests the exponent of the high-mass IMF to be approximately 1.6 if the UCDs are 13 Gyr old (i.e. almost as old as the Universe) or approximately 1.0 if the UCDs are 7 Gyr old, in contrast to 2.3 for the Salpeter-Massey IMF. If the IMF was as top-heavy as suggested here, the stability of the UCDs might have been threatened by heavy mass loss induced by the radiation and evolution of massive stars. The central densities of UCDs must have been in the range 10^6 to 10^7 Solar masses per cubic parsec when they formed with star formation rates of 10 to 100 Solar masses per year.

Effects of rotation on the evolution of primordial stars

(Abridged) Rotation has been shown to play a determinant role at very low metallicity, bringing heavy mass loss where almost none was expected. Is this still true when the metallicity strictly equals zero? The aim of our study is to get an answer to this question, and to determine how rotation changes the evolution and the chemical signature of the primordial stars. We have calculated 14 differentially-rotating and non-rotating stellar models at zero metallicity, with masses between 9 and 200 Msol. The evolution has been followed up to the pre-supernova stage. We find that Z=0 models rotate with an internal profile Omega(r) close to local angular momentum conservation, because of a very weak core-envelope coupling. Rotational mixing drives a H-shell boost due to a sudden onset of CNO cycle in the shell. This boost leads to a high 14N production. Generally, the rotating models produce much more metals than their non-rotating counterparts. The mass loss is very low, even for the models that reach the critical velocity during the main sequence. Due to the low mass loss and the weak coupling, the core retains a high angular momentum at the end of the evolution. The high rotation rate at death probably leads to a much stronger explosion than previously expected, changing the fate of the models. The inclusion of our yields in a chemical evolution model of the Galactic halo predicts log values of N/O, C/O and 12C/13C ratios of -2.2, -0.95 and 50 respectively at log O/H +12 = 4.2.

Nucleosynthesis from massive stars 50 years after B2FH

We review some important observed properties of massive stars. Then we discuss how mass loss and rotation affect their evolution and help in giving better fits to observational constraints. Consequences for nucleosynthesis at different metallicities are discussed. Mass loss appear to be the key feature at high metallicity, while rotation is likely dominant at low and very low metallicities. We discuss various indications supporting the view that very metal poor stars had their evolution strongly affected by rotational mixing. Many features, like the origin of primary nitrogen at low metallicity, that of the C-rich extremely metal poor halo stars, of He-rich stars in massive globular clusters, of the O-Na anticorrelation in globular clusters may be related to the existence of a population of very fast rotating metal poor stars that we tentatively call the {\it spinstars}. A fraction of these {\it spinstars} may also be the progenitors of GRB in very metal poor regions. They may avoid pair instability explosion due to the heavy mass loss undergone during their early evolutionary phases and be, dependent on their frequency, important sources of ionising photons in the early Universe.

Massive Stars as Cosmic Engines Through the Ages

AbstractSome useful developments in the model physics are briefly presented, followed by model results on chemical enrichments and WR stars. We discuss the expected rotation velocities of WR stars. We emphasize that the (C+O)/He ratio is a better chemical indicator of evolution for WC stars than the C/He ratios. With or without rotation, at a given luminosity the (C+O)/He ratios should be higher in regions of lower metallicity Z. Also, for a given (C+O)/He ratio the WC stars in lower Z regions have higher luminosities. The WO stars, which are likely the progenitors of supernovae SNIc and of some GRBs, should preferentially be found in regions of low Z and be the descendants of very high initial masses. Finally, we emphasize the physical reasons why massive rotating low Z stars may also experience heavy mass loss.

Stellar population synthesis of post-AGB stars: the s-process in MACHO 47.2496.8

Context. The low-metallicity RV Tauri star MACHO 47.2496.8, recently discovered in the Large Magellanic Cloud, is highly enriched in carbon and heavy elements produced by the slow neutron capture process (s-process), and is most probably a genuine post-C(N-type) asymptotic giant branch (AGB) star. The intrinsic interpretation of the enrichement is further strengthened by detection of a significant infrared excess. The circumstellar dust is the relic of a recent episode of heavy mass loss. We use the analysis of the abundances of MACHO 47.2496.8 to constrain free parameters in AGB models. Aims. We test which values of the free parameters describing uncertain physical mechanisms in AGB stars, namely the third dredgeup and the features of the 13 C neutron source, produce models that better match the abundances observed in MACHO 47.2496.8. Methods. We carry out stellar population synthesis coupled with s-process nucleosynthesis using a synthetic stellar evolution code. Results. The s-process ratios observed in MACHO 47.2496.8 can be matched by the same models that explain the s-process ratios of Galactic AGB and post-AGB stars of metallicity >Z� /10, except for the choice of the effectiveness of 13 C as a neutron source, which has to be lower by roughly a factor of 3 to 6. The less effective neutron source for lower metallicities is also required when comparing population synthesis results to observations of Galactic halo s-enhanced stars, such as Pb stars. The 12 C/ 13 C ratio in MACHO 47.2496.8 cannot be matched simultaneously and requires the occurrence of extra-mixing processes. Conclusions. The confirmed trend of the decreased efficiency of the 13 C neutron source with metallicity requires an explanation from AGB s-process models. The present work is to date the first comparison between theoretical models and the detailed abundances of an extragalactic post-AGB star.

Substructure around M31: Evolution and Effects

We investigate the evolution of a population of 100 dark matter satellites orbiting in the gravitational potential of a realistic model of M31. We find that after 10 Gyr, seven subhalos are completely disrupted by the tidal field of the host galaxy. The remaining satellites suffer heavy mass loss and overall, 75% of the mass initially in the subhalo system is tidally stripped. Not surprisingly, satellites with pericentric radius less than 30 kpc suffer the greatest stripping and leave a complex structure of tails and streams of debris around the host galaxy. Assuming that the most bound particles in each subhalo are kinematic tracers of stars, we find that the halo stellar population resulting from the tidal debris follows an r^{-3.5} density profile at large radii. We construct B-band photometric maps of stars coming from disrupted satellites and find conspicuous features similar both in morphology and brightness to the observed Giant Stream around Andromeda. An assumed star formation efficiency of 5-10% in the simulated satellite galaxies results in good agreement with the number of M31 satellites, the V-band surface brightness distribution, and the brightness of the Giant Stream. During the first 5 Gyr, the bombardment of the satellites heats and thickens the disk by a small amount. At about 5 Gyr, satellite interations induce the formation of a strong bar which, in turn, leads to a significant increase in the velocity dispersion of the disk.

Infrared stellar populations in the central parts of the Milky Way galaxy

Near- and mid-IR survey data from DENIS and ISOGAL are used to investigate the structure and formation history of the inner 10 degree (1.4 kpc) of the Milky Way galaxy. Synthetic bolometric corrections and extinction coefficients in the near- and mid-IR are derived for stars of different spectral types, to allow the transformation of theoretical isochrones into observable colour-magnitude diagrams. The observed IR colour-magnitude diagrams are used to derive the extinction, metallicity and age for individual stars. The inner galaxy is dominated by an old population (> 7 Gyr). In addition, an intermediate-age population (200 Myr to 7 Gyr) is detected, which is consistent with the presence of a few hundred Asymptotic Giant Branch stars with heavy mass loss. Furthermore, young stars ( 1 kpc from the galactic centre, near the inner Lindblad resonance. The innermost part of the Bulge, within about 1 kpc from the galactic centre, seems azimuthally symmetric.

Structure and Evolution of Planetary Nebula Haloes

The density structure of the extended haloes of Planetary Nebulae (PN) is generally believed to reflect the previous history of heavy mass loss during the final stages of stellar evolution on the asymptotic giant-branch (AGB). In this review, we discuss different interpretations of the observed PN halo structures in the light of recent numerical simulations combining detailed AGB and post-AGB stellar evolution calculations with time-dependent hydrodynamical wind models.

On Dead OH/IR Stars

Because the net duration of all 1612 MHz emission phases from |b| > 10° OH/IR stars is ~1700 yr, one death, as evidenced by the disappearance of all 1612 MHz masers, should occur in a sample of 170 every 10 yr, on average, if they have only one emission phase. We report here on the reobservation after 12 yr of the 328 OH/IR stars in the Arecibo sky with a peak first-epoch I1612 > 100 mJy. Four of these now have undetectable 1612 MHz masers, while those from a fifth, FV Boo, appear to be in terminal decline. The blue IR colors, 315 day period of FV Boo, and presence of water and SiO masers in several of the newly identified dead OH/IR stars all suggest that these objects are still asymptotic giant branch (AGB) stars rather than proto-planetary nebulae. This conclusion suggests that most oxygen-rich AGB stars pass through the OH/IR star phase more than once. The positions of the dead OH/IR stars on first-epoch plots of I1612 versus S(25) and on plots of the ratio of these quantities versus IR color are entirely normal. However, all of the dead OH/IR stars have small, less than 12 km s-1, expansion velocities, and all but one have blue IR colors. When these criteria are used to further delimit the sample, the five dead OH/IR stars in a sample of 112 imply a mean 1612 MHz emission life te ~ 314 (+387, -97) yr. The short duration of this emission phase is readily understood if oxygen-rich AGB stars toward the end of the AGB pass through a brief OH/IR star phase after a thermal pulse, when, as often happens, they are not bright enough to support heavy mass loss on the luminosity ascent to a thermal pulse. Copious AGB mass loss is thus frequently triggered by events that follow a thermal pulse.

M giants in MACHO, DENIS and ISOGAL

A complete sample of 174 M giants classified by Blanco (1986) and later than subtype M0 in the NGC6522 Baade's Window clear field has been investigated to establish some general properties of cool Bulge stars. Photometric information has been obtained from the MACHO database to search for variability and, where possible, to determine periods. Near- and mid-IR magnitudes have been extracted from DENIS and ISOGAL. 46 semi-regular variables and 2 irregulars were found a mongst the 174. Many M5 and all stars M6 or later show variation, whereas earlier subtypes do not. The DENIS I-J and J-K_S colours and the luminosities of the M stars increase with M sub-class. K tends to increase with log P among M-type SR variables. Almost all the variables were detected at 7 microns during the ISOGAL programme. Excess radiation at 15 microns, indicative of heavy mass loss, is associated with very high luminosity and late spectral type. The limit of sensitivity of the ISOGAL survey was such that the non-variables were not detected. Four probable M stars not listed by Blanco (1986), two of which are semi-regular variables, were detected by ISOGAL. In the case of doubly-periodic SR variables, the longer periods have K mags which place them close to the D line of Wood (2000) in a K,log P diagram. The unusual MACHO light curve of one particular star, Blanco 26, shows the commencement of a long-period variation with an anomalously short and sharp event and appears to rule out a pulsational model for this phenomenon.

Detection of Massive Tidal Tails around the Globular Cluster Palomar 5 with Sloan Digital Sky Survey Commissioning Data

We report the discovery of two well-defined tidal tails emerging from the sparse remote globular cluster Palomar 5. These tails stretch out symmetrically to both sides of the cluster in the direction of constant Galactic latitude and subtend an angle of 26 on the sky. The tails have been detected in commissioning data of the Sloan Digital Sky Survey, providing deep five-color photometry in a 25-wide band along the equator. The stars in the tails make up a substantial part (~) of the current total population of cluster stars in the magnitude interval 19.5 ≤ i* ≤ 22.0. This reveals that the cluster is subject to heavy mass loss. The orientation of the tails provides an important key for the determination of the cluster's Galactic orbit.

On the Transience of Circumstellar Shells about |b| ≥ 10o OH/IR Stars. I. Basic Statistics

There are 62 OH/IR stars in the Arecibo sky with |b| ≥ 10°, |l| < 90°, and a 25 μm flux S(25) ≥ 2 Jy. These were identified as a result of a complete OH survey of color-selected IRAS sources, which also identified four O-rich proto-planetary nebulae (PPNs). Since OH/IR stars evolve into PPNs, the ~103 yr expansion age of one, 18095+2704, calibrates an ~1670 yr average net duration for 1612 MHz emission from high-latitude stars. This is comparable with the 770 yr wind travel time from the photosphere out to the 1612 MHz emission zone of WX Psc and the ~500 yr duration of the luminosity spike following after a thermal pulse. There is, however, only one PPN candidate without an OH maser to match to the sample's 118 OH/IR-star color mimics (circumstellar shells with the IR colors of OH/IR stars that do not exhibit 1612 MHz masers), where eight are implied if mimics independently evolve through a PPN phase. Most mimics are therefore likely to be the immediate precursors of OH/IR stars. The average net duration of heavy mass loss in a superwind is then estimated from the ratio between the number of IR sources and PPNs at ~3700 yr. However, the absence of any O-rich PPNs to match to OH/IR stars and mimics in the Galactic anticenter region (AGC), even though there are two carbon stars and four C-rich PPN with appropriate IR colors there, offers a strong hint that most OH/IR stars in the AGC exit their asymptotic giant branch phase as C-rich PPNs. This carries the direct implication that the superwind phase in high-latitude stars is usually a cyclical phase.

Compact circumstellar dust envelopes of evolved AGB stars with heavy mass loss

With the use of ISOPHOT on board ISO, we have carried out far-infrared mapping observations of the circumstellar dust envelopes around evolved AGB stars IRC—10529 and WX Psc which have an optically very thick circumstellar dust envelope. The radial sizes of their dust envelopes are very small indicating that the period of heavy mass loss which made the dense dust envelopes is not long. It is very likely that IRC—10529 and WX Psc have recently evolved into the final stage of AGB evolution with an extremely high mass loss rate.

Miras and Mass Loss in the Local Group

Evolved AGB stars pass through a phase of heavy mass loss during which they become optically obscured. We describe an observational program to find and study such stars in the Magellanic Clouds. Many of the obscured stars are found to be luminous carbon stars, a population previously missing. We discuss the Mira PL relation, valid before the onset of the heaviest mass loss. We find that a number of semi-regulars fit this relation, based on Hipparcos parallaxes. No evidence is seen for a Z-dependence, giving a powerful potential distance indicator. We derive a distance modulus to the LMC of m − M = 18.63 ± 0.09 as well as a bright calibration for the Horizontal Branch in globular clusters.

Discovery of Infrared Stars in Globular Clusters in the Magellanic Clouds and Their Light Variations

A systematic near-infrared survey was made for globular clusters in the Magellanic Clouds. Two infrared stars were discovered in NGC419 (SMC) and NGC1783 (LMC). NGC419 and NGC1783 are well-studied rich globular clusters whose turn-off masses and ages are estimated MTO ~ 2.0 Mʘ and т ~1.2 Gyr for NGC419, and MT0 ~ 2.0 Mʘ and т ʘ 0.9 Gyr for NGC1783, respectively.The periods of the infrared light variations were determined to be 540 dfor NGC419IR1 and to be 480 d for NGC1783IR1, respectively. Comparison of the measurements with the period—if magnitude relation for carbon Miras in the LMC by Groenewegen and Whitelock(1996) revealed that the Kmagnitudes of the infrared stars were fainter by about 0.3 — 0.8 magnitude than those predicted by the P — K relation. This deviation can be explained if the infrared stars are surrounded by thick dust shells and are obscured even in the K band. The positions of NGC419IR1and NGC1783IR1 on the P — K diagram suggest that AGB stars with the main sequence masses of about 2 Mʘ start their heavy mass-loss when P ʘ 500 d.

3.20. OH/IR stars as signposts for ancient starburst activity in the Galactic center

We have surveyed the Galactic center (GC) for OH/IR stars, evolved late-type stars of different masses - and thus ages - in a short-lived stage of heavy mass-loss. By observing the 1612 MHz OH masers generated in their circumstellar shells, it is found that a considerable fraction of these OH/IR stars has the same high-valued shell expansion velocity of 19 km s−1.

Masers: Probing the mass loss process in PPN

With the advent of the IRAS All-Sky Survey a sizeable number of transition objects between the AGB and the PN-phase were found - the Proto Planetary Nebulae (PPN). Oxygen-rich AGB stars often show prominent masers of SiO, H2O, and OH, which are lost during the transition process. The heavy mass loss on the AGB however does not stop abruptly and a new axisymmetric wind develops during the PPN phase. These winds both may host new masers and they can be used to study the changes of the mass loss process after that the stars have stopped their large-amplitude variations on the AGB. Several PPN are known to have OH masers, and at least in one case, HD 101584, the presence of a bipolar outflow could be proven (te Lintel Hekkert et al. 1992). Lewis (1989) found that main-line OH masers become prominent again. I will discuss here conclusions, which can be drawn from observations of H2O masers in PPN.

Model Atmospheres of AGB Stars: Dynamics, Molecules and Dust

The atmospheres and circumstellar envelopes of AGB stars are characterized by complex physical phenomena like shock waves caused by stellar pulsation or formation of molecules and dust which often lead to a heavy mass loss and have a strong influence on IR properties as observed by ISO. To allow a physical interpretation of various observations we have constructed improved dynamical model atmospheres of long-period variables. In this contribution we mainly investigate the dependence of the atmospheric structure and its variability on stellar pulsation, molecular opacities and time-dependent dust formation. IR spectra resulting from our models are discussed in detail by Loidl et al. (1997b) and compared to ISO-SWS spectra obtained by Hron et al. (1997).

Evolution of Massive Stars into Luminous Blue Variables and Wolf-Rayet Stars for a Range of Metallicities: Theory versus Observation

view Abstract Citations (90) References (60) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of Massive Stars into Luminous Blue Variables and Wolf-Rayet Stars for a Range of Metallicities: Theory versus Observation Stothers, Richard B. ; Chin, Chao-Wen Abstract Evolutionary tracks for 30-90 Msun stars, computed along the lines described in our recent series of papers, suggest that luminous blue variables (LBVs) are the blue remnants of a prior phase of heavy mass loss. They are now intermittently experiencing ionization-induced dynamical instability within their hydrogen-poor envelopes, while their massive helium cores are in an advanced stage of central helium burning. In the H-R diagram, our new evolutionary models successfully explain the high- and low- temperature limits, the low-luminosity limit, and the relative luminosity function of the observed LBVs at quiescence which have log (L/Lsun) < 6.3. Metal-poor LBVs are predicted (correctly, in the only case known) to be significantly cooler than LBVs with normal metallicities. In the (mass, luminosity) plane, the models and observed stars agree closely, and show no discernible metals dependence. Predicted values of the surface hydrogen abundance and of the expelled mass during the LBV phase match reasonably well the available observational data, which, however, are still very crude. The observed cycles of mass loss are predicted correctly. Pre-LBV and post-LBV stars can be identified with the hydrogen-poor and hydrogen-free WN stars, respectively. Post-red-supergiant lifetimes, derived from published star counts and kinematical ages of nebulae around the WN stars and LBVs, are used to infer an approximate mean value of the rate of stellar wind mass loss from luminous red supergiants. This parameter is crucial for understanding stellar evolution at log (L/Lsun) < 5.8. Stars more luminous may never become red, and may spend all, or nearly all, of their post-main-sequence lives as blue supergiants. Our theory of LBVs makes a number of other predictions that have not yet been tested. A critical comparison with the very different theory of Langer et al. is presented. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177740 Bibcode: 1996ApJ...468..842S Keywords: STARS: EVOLUTION; STARS: OSCILLATIONS; STARS: WOLF-RAYET; STARS: VARIABLES: OTHER LUMINOUS BLUE VARIABLES; STARS: SUPERGIANTS full text sources ADS | data products SIMBAD (14)